Stretching flow instabilities at the exits of extrusion dies

Cogswell, F.N.

doi:10.1016/0377-0257(77)80031-1

Cited by 178 publications

(79 citation statements)

References 10 publications

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“…12) visually resemble the sharkskin extrusion defect. Cogswell [103] proposed a sharkskin mechanism where the surface defects arise from the tearing and cracking of the material at the surface of the extrudate, just downstream of the die exit, when the skin layer is subjected to large tensile stresses due to the change of boundary condition. The experimental investigations conducted in [104,105] validate this hypothesis.…”

Section: Discussionmentioning

confidence: 99%

Numerical simulation of the planar extrudate swell of pseudoplastic and viscoelastic fluids with the streamfunction and the VOF methods

Comminal

Pimenta

Hattel

et al. 2018

Journal of Non-Newtonian Fluid Mechanics

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. Numerical simulation of the planar extrudate swell of pseudoplastic and viscoelastic fluids with the streamfunction and the VOF methods. Journal of Non-Newtonian Fluid Mechanics, 252, 1-18. DOI: 10.1016/j.jnnfm.2017 A B S T R A C TWe present an Eulerian free-surface flow solver for incompressible pseudoplastic and viscoelastic non-Newtonian fluids. The free-surface flow solver is based on the streamfunction flow formulation and the volume-of-fluid method. The streamfunction solver computes the vector potential of a solenoidal velocity field, which ensures by construction the mass conservation of the solution, and removes the pressure unknown. Pseudoplastic liquids are modelled with a Carreau model. The viscoelastic fluids are governed by differential constitutive models reformulated with the log-conformation approach, in order to preserve the positive-definiteness of the conformation tensor, and to circumvent the high Weissenberg number problem. The volume fraction of the fluid is advected with a geometric conservative unsplit scheme that preserves a sharp interface representation. For the sake of comparison, we also implemented an algebraic advection scheme for the liquid volume fraction. The proposed numerical method is tested by simulating the planar extrudate swell with the Carreau, Oldroyd-B and Giesekus constitutive models. The swell ratio of the extrudates are compared with the data available in the literature, as well as with numerical simulations performed with the open-source rheoTool toolbox in OpenFOAM ® . While the simulations of the generalized Newtonian fluids achieved mesh independence for all the methods tested, the flow simulations of the viscoelastic fluids are more sensitive to mesh refinement and the choice of numerical scheme. Moreover, the simulations of Oldroyd-B fluid flows above a critical Weissenberg number are prone to artificial surface instabilities. These numerical artifacts are due to discretization errors within the Eulerian surface-capturing method. However, the numerical issues arise from the stress singularity at the die exit corner, and the unphysical predictions of the Oldroyd-B model in the skin layer of the extrudate after the die exit, where large extensional deformations occur.

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Section: Discussionmentioning

confidence: 99%

Numerical simulation of the planar extrudate swell of pseudoplastic and viscoelastic fluids with the streamfunction and the VOF methods

Comminal

Pimenta

Hattel

et al. 2018

Journal of Non-Newtonian Fluid Mechanics

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“…The dimensional change can be explained by several causes including an inhomogeneous elastic response of the melt to flow fields. 6 High molecular portion such as long chain branch (LCB) and crosslinking has displayed significant effect on the elastic response and critical shear stress with melt fracture. 7 9 Melts of linear polymers exhibit extrusion instability, 10 known as melt fracture, in which there are two critical stresses.…”

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confidence: 99%

Effect of Peroxide Modification on Melt Fracture of Linear Low Density Polyethylene during Extrusion

Kim

Yang

1999

Polym J

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ABSTRACT:A commercial linear low density polyethylene (LLDPE) was modified by adding a small amount dicumyl peroxide (DCP) during extrusion at moderate processing temperature (200°C). The effects of peroxide modification were to change the molecular weight and rheological properties, as evident by increase in high molecular portion, shear sensitivity and melt viscosity at low frequency region. These effects were confirmed by gel permeation chromatography (GPC) and Rheometric dynamic spectrometer (RDS). One outstanding feature of peroxide modification was the remarkable delay in the onset of melt fracture upon extrusion. The dilution of DCP in polypropylene (PP) powder was more effective for improvement of these properties than by direct loading of DCP. The observed remarkable delay in melt fracture is believed to be due to increase in high molecular portion by producing long chain branch and crosslinking. That is, DCP modification results in delay in the onset of melt fracture through increase in high molecular portion which can resist high elongational stress when leaving the die.KEY WORDS Peroxide Modification ! Linear Low Density Polyethylene I Long Chain Branch I Melt Fracture I For more than 30 years it has been known that a polyethylene melt passing through a die may exhibit flow instabilities during constant speed extrusion. 1 5 These phenomena, visually observed as distortions of the extrudate, are serious problem in polymer processing, limiting the production rates and physical properties of final products. Extrudate distortions range from sharkskin at relatively low flow rates to large amplitude quasiperiodic oscillation in the extrudate diameter and gross melt fraction (wavy melt fracture) at higher extrusion rates. Extrudate distortions are identified as irregular dimensional change in the molten polymer. The dimensional change can be explained by several causes including an inhomogeneous elastic response of the melt to flow fields. 6 High molecular portion such as long chain branch (LCB) and crosslinking has displayed significant effect on the elastic response and critical shear stress with melt fracture. 7 9 Melts of linear polymers exhibit extrusion instability, 10 known as melt fracture, in which there are two critical stresses. The first critical stress is characterized by the visual appearance of regular fine-scale surface distortions known as sharkskin. In the second critical stress, there is an apparent discontinuity in the flow curve, sharkskin disappears, and the extrudate becomes wavy or distorted. This transition is known as wavy fracture or gross melt fracture. For polyethylenes such as linear low density polyethylene (LLDPE) and high density polyethylene (HDPE), fine-scale surface distortion (sharkskin) usually occurs before wavy melt fracture. However, highly branched polyethylene such as low density polyethylene (LDPE) have strong strainhardening elongational behavior and do not exhibit sharkskin defect or surface irregularities even though sharkskin has been observed by White et al. ...

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“…It is well-known that flow-induced alignment can alter the rheological properties in their molten state. 9,[11][12][13] Linear low density polyethylene (LLDPE) is extensively used in plastic markets using extrusion process such as films, electrical insulation material, and so on.14,15 Of these applications, power cables especially require insulation material that exhibits good surface roughness. There are several causes for surface roughness in extrusion process.…”

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confidence: 99%

Effect of Maleated Polyethylene on the Rheological Properties of LLDPE/Clay Nanocomposites

Kim¹,

Lee

Kim

et al. 2005

Polym J

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Stretching flow instabilities at the exits of extrusion dies

Cited by 178 publications

References 10 publications

Numerical simulation of the planar extrudate swell of pseudoplastic and viscoelastic fluids with the streamfunction and the VOF methods

Numerical simulation of the planar extrudate swell of pseudoplastic and viscoelastic fluids with the streamfunction and the VOF methods

Effect of Peroxide Modification on Melt Fracture of Linear Low Density Polyethylene during Extrusion

Effect of Maleated Polyethylene on the Rheological Properties of LLDPE/Clay Nanocomposites

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